Digital subscriber line (DSL) technologies can provide large bandwidth for digital communications over existing subscriber lines. When transmitting data over the subscriber lines, some DSL technologies, such as Asymmetric DSL (ADSL) or Very high rate DSL (VDSL), use a Discrete Multi-Tone (DMT) line code that allocates a plurality of bits for each sub-carrier or tone in each symbol. The DMT may be adjusted to various channel conditions that may occur at each end of a subscriber line. In such technologies, typically three power management states may be defined, such as in the Telecommunication Standardization Sector (ITU-T) G.992.3/5 and G.993.2 specifications for ADSL2 and VDSL2, respectively. The three power management states comprise a mode 0 (L0) state for normal operation mode, a mode 2 (L2) state for idle mode, and a mode 3 (L3) state for off mode.
Generally, the DSL link is idle most of time. L2 state is designed such that the transceivers can take advantage of that factor to reduce the power consumption by reducing the transmitted signal level. Hence, the DSL transceiver should be maintained more frequently in the L2 state rather than the L0 state at a relatively low-speed connection, which reduces the transmission power spectrum density (PSD) at the corresponding line. Reducing the transmission PSD reduces the power consumption of the line driver, which comprises a relatively large portion of the total power consumption in the system. Additionally, reducing the transmission PSD reduces the crosstalk interference between transmitted signals over adjacent twisted-pair phone lines in a same or nearby bundle of lines. The DSL devices on those lines may sense the decrease in crosstalk interference and adjust their operations accordingly, for instance by increasing their data rate or reducing their transmission PSD. However, when the DSL link is switched from the L2 state to the L0 state to start or resume normal operation mode, the transmission PSD at the corresponding line may suddenly increase. Consequently, the crosstalk interference between signals over the adjacent lines, which have adjusted operations in response to prior decrease in adjacent transmission PSD, may suddenly increase creating non-stationary crosstalk that affects proper reception of signals and degrade stability of DSL operations. Due to this reason, although L2 state is defined in the ADSL2 specifications, the operators are reluctant to use it, and it is not yet defined in VDSL2.